Structure-evolution-designed amorphous oxides for dielectric energy storage

Recently, rapidly increased demands of integration and miniaturization continuously challenge energy densities of dielectric capacitors. New materials with high recoverable energy storage densities become highly desirable. Here, by structure evolution between fluorite HfO2 and perovskite hafnate, we create an amorphous hafnium-based oxide that exhibits the energy density of ~155 J/cm3 with an efficiency of 87%, which is state-of-the-art in emergingly capacitive energy-storage materials. The amorphous structure is owing to oxygen instability in between the two energetically-favorable crystalline forms, in which not only the long-range periodicities of fluorite and perovskite are collapsed but also more than one symmetry, i.e., the monoclinic and orthorhombic, coexist in short range, giving rise to a strong structure disordering. As a result, the carrier avalanche is impeded and an ultrahigh breakdown strength up to 12 MV/cm is achieved, which, accompanying with a large permittivity, remarkably enhances the energy storage density. Our study provides a new and widely applicable platform for designing high-performance dielectric energy storage with the strategy exploring the boundary among different categories of materials.

1) The authors claimed that the energy density achieved in the "amorphous" hafnium oxide is record-high in high-k materials. Probably, this statement is inappropriate. In general, SrTiO3 and BaTiO3 can also be classified into high-k materials, where higher energy densities have been reported, as listed in Table S1. In Fig. 4e, the binary oxides, such as HfO2 and ZrO2 systems, may be described as the high-k materials that are easily compatible with the current CMOS process. It is better emphasize this point clearly.
2) In the BHO02 thin film, the authors have described the coexistence of m-and o-phases. However, in the HAADF image, I only found the results of o-phase. The authors should provide the STEM results for the m-phase of the BHO02. 3) Also, in the structure characterizations in Fig. 1, I found that the amorphous regions of Ba-Hf-O are strongly non-stoichiometric. Noting that these samples have also been treated by the same annealing temperature for the crystalline HfO2 and BaHfO3. Why are there no secondary or impurity phases formed in the structural evolution range? 4) In the amorphous structure, the metal-oxygen bonding may differ from that of the crystalline form. It is thus necessary to characterize the valence state of Hf and Ba ions. 5) In Fig. 3a, compared with the BHO02, I found that the hysteresis of BHO04 capacitor becomes weaker but this composition exhibits a more pronounced ferroelectric o-phase in the XRD patterns. Why? 6) About the discussion of dielectric breakdown properties, it is better to measure leakage currents of the BHO capacitors as well, and include it in the paper. 7) It is also related to the discussion of breakdown. In this paragraph, the wording "Considering that the BHO thin films……, the electronic breakdown……can be excluded" is inappropriate. In the BHO capacitors, the electronic breakdown cannot be excluded since it is an intrinsic character of dielectrics. In fact, what could be excluded in this case is that the electronic breakdown may be not a dominant factor for the difference in breakdown strength between the amorphous and crystalline BHO capacitors. 8) Regarding the analysis of breakdown field with thickness, the negative power law used in Fig. S10 is a general description for breakdown behaviors of dielectrics. I suggest the authors employing specific formula of the avalanche mechanism to analyze the thickness dependent breakdown strength, such as the "40-generation-electron theory" mentioned in Ref. 1, which probably be better. 9) For the calculation of formation of oxygen vacancy, there are oxygen rich and poor conditions. It seems like that the oxygen poor condition are considered. How does that calculation match the experimental environment?
The manuscript by Yu et al. reported amorphous hafnium-based oxides for dielectric energy storage. They created amorphous hafnium-based oxides by bridging fluorite HfO2 and perovskite hafnate. The structure evolution with different compositions was investigated by XRD and TEM. The formation mechanism of amorphization was tried to understand by DFT calculations. A breakdown field strength (Eb) of ~12 MV/cm and a recoverable energy storage density (Urec) of ~155 J/cm3 were successfully obtained in this material system. This work proposed a structure-evolution strategy to overcome the negative correlation between Eb and permittivity. It may guide the research of energy storage materials and expand the application field of hafnia-based oxides, while some scientific problems should be solved. Table S1, the thickness in different samples is varied. What's the influence of the thickness to the structure evolution and energy storage performance? 2. In Figure 1c, the diffraction peak at 30 degree was labeled as o-phase. But the diffraction peak of tetragonal (t-) phase is also around 30 degree. Why the authors exclude the possibility of t-phase in the film. Why the peak of BHO0 at 30 degrees is not significant, but the peak at 28 degree is more significant than that in BHO02 and BHO04. What does the peak at 42 degrees stands for, it seems that the intensity gradually reduced from bottom (Fluorite HfO2) to top (Perovskite BaHfO3)? 3. The authors said that the amorphous BHO was formed in a high-temperature (973 K) crystallizing process and the HfO2 should have a high symmetry, like the cubic (c-). Then, they adopted c-phase in the DFT calculation (Fig. 1g, h). Based on the phase diagram of HfO2, c-phase is an ultra-high temperature phase. At 973K, the HfO2 should have a msymmetry without dopants and pressure, or have a t-symmetry with dopants or pressure. The authors need to reevaluate the calculation result. 4. In Figure 1e, small crystalline clusters in HAADF images and texture spot in FFT patterns can be found, did this suggest that the film is not in the amorphous state. Since the TEM image reflects a local area, more TEM evidences should be provided. Besides, the elemental distribution for Hf and Ba in Figure 1e seems also not very uniform, especially for Ba element as shown in Figure S5, is this the reason for the change in performance? 5. In Figure 1g, h and Figure S7, why did the authors only choose the c-phase, m-phase and o-phase in the DFT calculations. The t-phase should also be considered. 6. The two fitting curves for the EXAFS data do not match the experimental results very well, especially in the upper curves. 7. In the last paragraph of "Introduction", the authors took a lot of words to describe the crystal structure of HfO2 and AHfO3 and the strategy of the amorphous structure designing in Fig. 1a. These contents seem inappropriate to be described in "Introduction", but in the first part of "Results". Besides, there were many detailed and conclusive sentences about the physical mechanisms. However, these conclusions can only be reached after describing the experimental results and thus should be moved to the "Conclusions" or "Discussions" We appreciate your time and effort for reviewing our manuscript, and cordially thank you for your comments that the structure strategy proposed in this work is novel and provides a new perspective for material design in electronic fields. All your comments have been addressed point-by-point as shown below and the manuscript has been revised correspondingly. The revisions are highlighted in blue in the revised manuscript.

1) The authors claimed that the energy density achieved in the "amorphous"
hafnium oxide is record-high in high-k materials. Probably, this statement is inappropriate. In general, SrTiO 3 and BaTiO 3 can also be classified into high-k materials, where higher energy densities have been reported, as listed in Table   S1. In Fig. 4e, the binary oxides, such as HfO 2 and ZrO 2 systems, may be described as the high-k materials that are easily compatible with the current CMOS process. It is better to emphasize this point clearly.
In this work, we refer the high-κ materials to the HfO 2 -based (including ZrO 2 -based) binary oxides that could be compatible with the current CMOS process and are also utilized in commercial devices. We therefore classified SrTiO 3 and BaTiO 3 into the paraelectric and ferroelectric perovskites. However, in a broad sense, the high-κ terminology can also mean the material having permittivity higher than that of SiO 2 . We appreciate the reviewer for pointing out this.
According to the comment, we have deleted the "… is record-high in high-κ materials…" wording in Abstracted section and revised the "high-κ materials" to the "high-κ binary oxides", which is distinguished from the perovskite oxides, throughout the manuscript. In addition, we also revised Table S1, in which the materials are classified into two major groups, the high-κ binary oxides, including 3) Also, in the structure characterizations in Fig. 1 It is indeed an interesting phenomenon in our samples. We also noticed this point when preparing the manuscript. We think that the absence of impurity phases might be ascribed to the maintaining of face-centered metal frames during the structure evolution since the structure similarity of the patent HfO 2 and BaHfO 3 .
To evidence this hypothesis, we carried out an additional experiment by annealing the BHO12 sample with further elevating temperature, as shown in      Thank you very much for pointing out our mistake in the discussion. We have revised the discussion accordingly, as shown in line 3-6, page 14.

How does that calculation match the experimental environment?
As the reviewer stated, for the chemical potential of oxygen, two cases are considered: oxygen rich and oxygen poor. The calculated under oxygen rich condition is the upper limit while the under oxygen poor condition is the lower limit. Fig. R4 demonstrates the of the first nearest-neighbor of t-phase HfO 2 (as suggested by the reviewer #2) for both oxygen rich and poor conditions. In fact, the experimental environment of our work is between oxygen poor and rich. Therefore, we provided the under oxygen rich condition in the manuscript for ensuring the validity of the calculation results.
We have added the explanation of oxygen condition adopted in the first-principles calculation to the Methods section, as shown in line 1-2, page 19.  Table S1, the thickness in different samples is varied. What's the influence of the thickness to the structure evolution and energy storage performance?

According to the comment, we have deposited the Ba-Hf-O thin films with
representative compositions of 4%, 6%, 12%, 15%, and 20% around the amorphous region in the thickness of 50 nm, as shown in Fig. R1. As shown, the 50 nm-thick BHO exhibits the same amorphous region of 4% < x < 20% with that of the 30 nm-thick BHO (Fig. 1c). The only difference is that the 50 nm-thick BHO04 film has more m-phase, as evidenced by the pronounced Bragg reflection at 2θ = 34.2°. In addition, we also deposited a 10 nm-thick BHO12 thin film, which is also in the amorphous state, as shown in Fig. R2. Therefore, one may draw a conclusion that the film thickness has little effects on the structure evolution in our material design strategy.
In Fig. R3 Figure 1c, the diffraction peak at 30 degree was labeled as o-phase. But the diffraction peak of tetragonal (t-) phase is also around 30 degree. Why the authors exclude the possibility of t-phase in the film. Why the peak of BHO0 at 30 degree is not significant, but the peak at 28 degree is more significant than that in BHO02 and BHO04. What

STO
Thank you for the comments. We agree with the reviewer that the t-phase may also coexist with the o-phase since the (011) t reflection is located at 30.05° and the 2θ difference between (111) o and (011) t is too small to be distinguished. We therefore added the discussion to the main text, as shown in line 14-16, page 7. We also revised Fig. 1c by adding the indication of t-phase at 2θ of ~30°.
In Fig. 1c, the diffraction peak at 2θ of ~30° is o-phase HfO 2 (or with t-phase). The presence of o-phase is due to the Ba substitution-induced lattice distortion.
Therefore, one can find the o-phase peaks in BHO02 and BHO04 thin films. For the BHO0, it is the undoped HfO 2 thin film, which is in the m-phase, the energetically-favorable phase of bulk HfO 2 , since there is no substitution-induced lattice strain. The Bragg reflection for m-phase HfO 2 is at 2θ of ~28°. Therefore, one can find in the XRD patterns that the BHO0 exhibits a significant peak at ~28° while the BHO02 and BHO04 exhibit significant peaks at ~30°. According to the comment, we are aware that the abbreviation BHO0 may be somewhat misleading.
We therefore added an explanation of BHO0 when it first appears, as shown in line 10, page 7 in the main text.
The peak at 2θ of ~42° is k β of STO substrate. We have indicated it in Fig. 1c and added a description in the caption.

The authors said that the amorphous BHO was formed in a high-temperature
(973 K) crystallizing process and the HfO 2 should have a high symmetry, like the cubic (c-). Then, they adopted c-phase in the DFT calculation (Fig. 1g, h).  Figure 1e, small crystalline clusters in HAADF images and texture spot in FFT patterns can be found, did this suggest that the film is not in the amorphous state. Since the TEM image reflects a local area, more TEM evidences should be provided. Besides, the elemental distribution for Hf and Ba in Figure 1e seems also not very uniform, especially for Ba element as shown in Figure S5, is this the reason for the change in performance?

In
In the HAADF image of BHO12 (Fig. 1e)  In the fitting of EXAFS data, the fitting window is set to R = 1.0 ~ 4.0 Å, which is typical for the EXAFS analysis of amorphous structure because (i) R < 1.0 Å is meaningless since there is no interatomic distance shorter than 1.0 Å in actual crystals; (ii) the oscillation amplitude of EXAFS spectrum is too low to be fitted when R > 4.0 Å since the amorphous structure only has short-range ordering.
Within the fitting window, the fitting of EXAFS data in Fig. 2 is good with very low R-factor of 0.0065 for the BHO12-RT (the inset) and 0.0092 for the BHO12.
To make the representation more clearly, we have added the indication about the fitting window in Fig. 2. The explanation for the set of fitting window is also added to Supplementary Text 1.
7. In the last paragraph of "Introduction", the authors took a lot of words to describe the crystal structure of HfO 2 and AHfO 3 and the strategy of the amorphous structure designing in Fig. 1a. These contents seem inappropriate to be described in "Introduction", but in the first part of "Results". Besides, there were many detailed and conclusive sentences about the physical mechanisms.
However, these conclusions can only be reached after describing the experimental results and thus should be moved to the "Conclusions" or Finally, we thank the reviewer again for the invaluable suggestions and comments.
The manuscript is indeed improved significantly after the revision. We hope this revised manuscript would meet the criteria for publication in Nature